Radio receiving system



Jan. 31, 1933. v c MACNABB RADIO RECEIVING SYSTEM Filed July 23, 1930Patented Jan. 31, 1933 UNITED -s;'r Es PATENT -OFFICE VERNON C.,MAGNABIB, OF PHILADELIHIA, PENNSYLVANIA, ASSIGNORUJO A'I'WA'IEB KENTMANUFACTURING COMPANY, OF PHILADELPHIA, PENNSYLVANIA, A COR- QPORATIONOF PENNSYLVANIA nnmo RECEIVING SYSTEM v Application filed July 23,

tion of the desired signal input voltage, and

particularly without such attenuation of desired signal input voltage asarises from m- "crease in number of the cascaded circuits, tunable tothe desired signal frequency, to the number necessary to effectcomparable discrimination; and it is a further object of my invention toeffect discrimination-of the character aforesaid in addition to thateffected by a circuit itself modified suitably to effect furtherdiscrimination against undesired signal frequency beyond th effected bytuning it to the desired signal frequency; A selector system of thischaracter in which marked discrimination is made between the desired andundesired signals is particularly desirable and of great advantage insuperheterodyne systems in which selection of the desired signal iseffected-in advance of the first detector and a selective intermediateamplifier follows the detector, particularly when the adjustable tuningelements of the selector system ahead of the first detector aremechanically coupled to each other andtothe frequency varying element ofthe oscillator, for movement in unison. My invention resides in a systemof the character hereinafter described and claimed; For an understandingof my invention and for an illustration of some-of the various forms itmay take, reference is to be had to the accompanying drawing, in which:Fig. 1 is a circuit diagram of a part of a superh'eterodyne receivingsystem embodying my invention; "1 1 Fig. 2 is a diagram of amodification.

' Referring to Fig. 1, V is the first detector 1930. Serial no. 470,014.

of a superheterodynereceiver. In its plate mg the desired signal, whichordinarily is v speech-or music."

The plate circuit of the detector V will be understood to be coupled toone or more stages of beat or intermediate frequency amplifiers of thethermionic; type (preferably sharply tuned to a narrow band offreuencies including the intermediate or beat requency), followed by asecond detector delivering signal representing current to a signaltranslating instrument, commonly a loud speaker, either directlyorthrough one or more intervening audio frequencythermionicam lifiers. 7

i The etector V is of the thermionic type having the control grid 9,anode or plate a and cathode 0 maintained at suitabletemperature by theelectric heater h. In the present example the tube V is of the shield orscreen grid type by virtue of the presence of the screen grid 6.It'sha'll'be understood, however, that any suitable type ofdetector maybe employed and that when ofthe thermionictype its cathode may consistof a filament heated by current passed therethrough.

Thelocal oscillations are produced by'the tube 0 of any suitable t pe,in the present example comprising a lament cathode f,

anode a and grid g. In its grid and plate circuits are the coils orinductances L and L1, coupled to each other. In a resonant loop with theinductance L is a variable condenser C for determining the'frequencyofthe OSCll-r lations generated. In the example illustrated tor system;This particular type of oscillator system is disclosed 'in Millerapplication,

Serial" No. "470,016, filed July 23, .1930. Or.

there may be employed the system of my copending application Serial No.470,557, filed July 25, 1930. Those systems have the characteristic'that the frequency of the oscillations locally generated isvaried substantially entirely in accord with variation of thecapacity'offthecondenser G ithatjthe amplitude of the oscillations issmall; and that the voltage impressed upon the input of the detector"Via-substantially constant or. 'V'ar'ie's Within suitably narrow'li'niitscthr'oughout the range of frequencies of the generatedoscillations.

It will be understood, however, that my invention is not limited tothe-particular typeof oscillator employed, and any suitable generallyequivalent type is contemplated. The coilLQ, coupledto:the;coil L,.is ineffect the secondary of a transformer whose 'rimary is traversed by thegenerated. oscillations. i One terminal of the secondary LQisflconne'cted to the cathode of the detector ,V, and the other,preferably through a' grid I biasing resistance R1 shunted by acondenser an alternating'current circuit indicated the conductors 1 and2, across which is connected the primary of a transformer T havingthesecondary S whose terminals are connectedto the; anodes a of therectifier tube V1, having the filament or cathode fheated by currentfrom thesecondary $1. Theunhdirectional pulsating current deliveredthrough the rectifier tubeVl is smoothed or filtered by a filter systemcomprising, for example, inducfthrough a: choke coil. vv

.tances ,3, v4: and capacities 5 6, 7 The posi tive terminal" of theplategcircuit source is indicated at8, while ground or terminal .9 isthe'negati've pole of the source. Current from this source is suppliedto the plate circuits of the tubes 0, and theother ther- Inionic tubesof the system. The voltage effective in the plate'circuit ofthe-oscillator O is determined by the resistance B2 shunted by acondenser K3. In the example illustrated the plate @of thefirstldetectortube V is Connected to the, terminal 8,, preferably Thetransformer T is'providedlwith one or more secondaries S2 'supplyingcurrent for heating the cathodes of the thermionic tubes of the set; Q

' iThe coil L2 is in effect connected in the cathode lead of the tube-Yextending to one terminal of a loop circuit tunable to the variousfrequencies of the desired signals,

I The loop circuit comprises the secondary 8 inseries with theadditional inductance s1,

that series combination shunted'by the vari- .603, filed'July '3,1930;11; has the desirable characteristic of affording addiitional-sdissV crimination against-signals of undesired frequencies, particularlythose differing in fixed amount from the desired signal frequency,- andmore particularlydiffering from the desired frequency by twice thefrequency ofthe locally generated oscillations.

Coupled to. the secondary is the primary p in series with the inductanceL3; Theseine ductances in series are bridged the con; denser C3,variableto tune'the loop so formed to the desired signalfrequency. 1 0

The absorption or a'ntenna system, which 7 first appear oscillationsrepresentative of the signals is in the example illustrated coupled tothe loop L3, 1), C3 through the capacityKl of say 7 micro-microfarads,andcomprise's an open antenna path ofwhich D is the an; r

tenna proper connected to -earth or counter capacity E,through theinductance L4, con denser K2 preferably of fixed capacity, and

' the condenserC l variable to tune the absorption system to theundesired signal, more particularlyto the undesired frequency whichdiffers from the desired signal frequency by twice the frequency of'thelocally generated oscillations. a k Assuming the system'to be designedfor reception of broadcast signals having a frev quency anywhere withinthe range from 550 to 1 500 kilocycles, and the beat or intermediatefrequency of the superheterodyne system to be say 130kilocycles, thefrequency of the oscillations locally generated may be higher or lowerthan the signal frequencies lay-130 kilocycles. It will be assumed thatthe 10- cally generated frequency is higher thanthe desired signalfrequency,and in such case the localfrequency willvary between 680 and1630 kilocycles as determined by. the setting of the condenser C. Withinthe broadcast rangethere are other frequencies which ,in reaction withthe locally generated oscillw tions willalso produce the same heat orintermediate frequency ofl30 k c. This causes the so-called,double spotor image inter ference, because it is possibleunder the circumstancesmentioned that two stations may 7 be broadcasting at the same timesignal frewhen the undesired signal frequency is that ofelectro-ma-gnetic waves radiated from a nearby or powerful broadcastingstation while the desired signal is relatively weak.

The frequency of the undesired interfering signal may be higher thanthat of the desired signal by the aforesaid 2601:. c. It is to thatundesired frequency that the absorption path is tuned by variation ofthe condenser C4. WVhen the absorption structure is so tuned to theundesiredsignal by the series resonance combination described, there'isafforded additional discrimination against theundesired signal beyondthat effected by the above described system tunable to the desiredfrequency. The additional discrimination arises from the fact that whenthe absorption structure is tuned to resonance for the undesired signalthe impedance of the tuned system is extremely low, and the voltagerepresenting the undesired signal is extremely low, and in consequenceoscillations of undesired frequency appearing in the input system are ofextremely low amplitude, reducing to a minimum the ,double spotinterference effect. Whatever be the amplitude of the oscillations ofundesired frequency. reaching the loop system tunable by the condenser62 in spite of the discrimination by tuning to the desired signalfrequency, their effect is further reduced because as to them the seriesresonant system s1C2 has extremely low impedance,lwith correspondinglydecreased effect upon the grid 9 of the detector V.

At the same time, however, the impedance of the absorption path withregard to oscillations of the desired signal frequency is such that theywill'materially affect through the coupling K4 the input system of thedetector V. Throughout the frequency range to which the absorptionstructure is serially tuned forv the undesired frequencies, theimpedance of that path remains suitably high and to all intents andpurposes constant for the desired signal frequencies. 7

A system of this character has the advantage that the oscillations ofdesired signal frequency are not materially attenuated, or in any eventare not attenuated to anything like the degree occurring when, fordiscriminating to likeextent against undesired signal frequencies, asuflicient number cf loops tunable to the desired frequency and incascade in advance of the input system of the detector V is employed.While attenuation caused by such number ofcascaded circuitsmay becompensated for by adequate amplification by the intermediate frequencyamplifier, there may arise undesirable tube noises occasioned by thehigh degree of amplification required. My system avoids such difficulty.a

* In brief, in accordance with myinvention a high degree ofdiscrimination against'undesired signal frequencies is obtainedwithoutrecourse to. additionalloops, tunable to the deslred frequency, incascade in advance of the detector tube, which arrangement seriouslyattenuates the desired signal, oscilcoupled for adjustment in unison orbygunicontrol. This uni-control may, as indicated, 7 be extended toinclude the condenser Oaof the local oscillator system whose capacity isso varied, as the aforesaid tuning devices are adjusted, that the'frequency of the localj oscillations always differs by the same, and tothe necessary extent for maintaining conx stant the beat orintermediate'frequency. I

It will be understood thatthe capacities of the severalcondensers C, C2,C3 and C4 may have as to any two or more of them, ,or' as to all ofthem, the same characteristic as regards extent of change of capacityper unit of angular adjustment oftheir rotors. Any two or more or all ofthese'condensers may be of the straight line capacity, straight linefrequency, or straight line wave length type, or of a type having acompositecharacteristic.

It will be further understood that any two or more or all of thesecondensers may if suitable and desirable be comprised in a sin gle unitor gang, and in such case the rotors of the different condensers may bemounted side by side upon the same shaft which is rotated for effecting:the uni-control adjustment. 1 7 j i There is preferably included in theantenna circuit a condenser K2 which is of such value that with thecondenser C4 set for the same capacity as condensers C2 and C3, theantenna circuitis always tuned in series resonance to a signal 260kilocycles higher than the fre-' quency of thedesired signal; thecondensers G2 and C3 tuning their respective circuits to the frequencyof the desired signah there is employed an inductive coupling betweenthe antenna or absorption structure and the selector system in advanceof the tube V. I11 this instance there is taken-a connection through theprimary p3 loosely coupled to the secondary L3. This inductive couplingis utilized in lieu of the capacitative couplingK i, Fig. 1,particularly when the antenna is of high orhigher capacity than that forwhich the capacitative coupling is 1 In Fig. 2 is shown a'modificationin which 1 10 onddetector, and a local generator of oscillations, saidselector system comprising an antennafpath including an inductance 1n'se'ries- -with serially related capacities, one of which is variableand another larger than the maximum value of the variable capacityutilized in'tuning, tunable lto-undesired signal frequencies differingfrom the desired signal frequencies by twice the intermediate frequency,another component of said selector system tunable to the-desired signalfrequencies comprising a loop including inductance and a variabletuningcapacity, means for "impressing upon the'first detector the voltageacross a. terminal of said loop and a point dividing the loop inductanceinto components one of which in series with the variable capacity ofsaid loop constituting a series resonance combination tunable to saidundesired signal frequency, and means for cou- 'pling said antennapathto said other component of said selector system.

2. A superheterodyne receiving system comprising a selector system, afirst detector, ianintermediate frequency amplifier, a seconddetector,.and a local generator of oscillations, saidselector systemcomprising an antenna pathincluding an inductance in series withserially related capacities one of which is variable and another largerthan the maximum value of the variable capacity utilized in tuning,tunable to undesired signal frequencies differing from the desiredsignal frequencies bytwice the intermediate frequency, another componentof said selector system'tunable to the desired signal frequenciescomprising a loop in'cluding'inductance and a variable tuning capacity,means for impressing upon the first detector the voltage across aterminal of said loop and a point dividing the loop, inductance intocompo nents one of which in series with the variable capacity of saidloop constituting a series resonance combination tunable to saidundesired signal frequency, and means for capaci tatively coupling saidantenna path'to said other component of said selector system.

3. A superheter'odyne receiving system comprising a selector system, afirst detector,"

anintermediate frequency amplifier, a: second detector, and a lo'calgenerator of oscillations, said selector system comprising an antennapath including an inductance in series with serially related capacitiesone of A which is variable and another larger than the maximum Value ofthe variable. capacity utilized in tuning, tunable to undesired signalfrequencies differing from the desired signal frequencies by twice theintermediate frequency, another component of said selector systemtunable to the desired-signal frequencies comprising aloop includinginductance and a variable tuning capacity, .means for impressing uponthe first detector thevoltagc across .a terminal of said loop and apoint dividing the loop inductance into'components one of which inseries with the variable capacity of said loop constituting a seriesresonance combination tunable to said undesiredsignal frequency, andmeans for inductively'coupling said antenna path to said other componentof'said selector system.

pling adjustable elements of said variable capacities for adjusting themin unison, and means for loosely coupling said other component of saidselector systemto said antenna path by a smallcapacity. q

'5. A superheterodyne receiving system comprising a selector system, afirstdetector, an intermediate frequency amplifier, a seconddetecton-and a local generator of osoillations whose frequency isdetermined by a variable capacity, said selector system comprising anantenna path including aninductance in series with serially relatedcapacities, one of which is variable and another larger thantllQhlflXlIlllllTl value of the variable capacity utilized in tuning,tunable to undesired signal frequencies difiering from .4. Asuperheterodyne receiving system F the desired signal frequencies bytwice the whose frequency is determined by a variable capacity, saidselector system comprising an antenna path including an inductance inseries with serially related capacities, one of which is variable andanother larger than the maximum value of the variable capacity utilizedin tuning, tunable to undesired signal frequencies differing from thedesired signal frequencies by twice the intermediate frequency, anothercomponent of said selector system tunable by variable capacity to thedesired signal frequencies, and means for coupling the other componentof said selector system to said antenna path, and means for mechanicallycoupling the adjustable elements of the capacities of the selectorsystem and the variable capacity of said oscillator for adjusting themin unison.

7. A superheterodyne receiving system comprising a first detector, aninput system therefor tunable to the desired signal frequency andincluding a variable condenser, an open antenna system coupled to saidinput system and including in series an inductance in series with avariable condenser similar to and adjustable through substantiall thesame range of capacity as said first con enser, a local oscillatorsystem including a variable condenser, means mechanically coupling theadjustable elements of all of said condensers for movement in unison, anintermediate frequency amplifier, and a fixed capacity in series withsaid second condenser and said inductance and of such magnitude thatthroughout the range of movement of said variable condensers in unison,said second variable condenser, said fixed capacity and said inductanceare continually in series resonance for a frequency higher than thefrequency of the desired signal by twice the intermediate frequency,substantially to eliminate transfer to said input system of energy ofsaid higher frequency.

8. A superheterodyne receiving system comprising a first detector, aninput system therefor tunable to the desired signal :fre-

quency and including a variable condenser, a shunt path coupled to andin advance of said input system and comprising an inductance connectedin series with a variable condenser similar to and adjustable throughthe same range of capacity as said first condenser, a local oscillatorsystem having a variable condenser, means mechanically coupling theadjustable elements of all of said condensers for movement in unison, anintermediate frequency amplifier, and a fixed capacity in said path inseries with said inductance and second variable condenser thereof, andof such magnitude that said path, throughout the range of adjustment ofsaid condensers in unison, is continually tuned to series resonance fora frequency higher than the fre-' quency of the desired signal by twicethe intermediate frequency substantially to eliminate transfer to saidinput of energy ofzsaid higher frequency. a v V a v ,1 J 9. Asuperheterodyne system comprising a first detector, an input systemtherefor tunit coupling the adjustable elementsof said condensers formovement in unison,a local OSCll w lator system, an intermediatefrequency amplifier, and a fixed condenser connected: 1111 series withsaid inductance and said second variable condenser, and of suchmagnitude of capacity that said inductance and series connectedcondensers are continually in series resonance for a frequency-higherthan the desired frequency by twice the intermediate frequencysubstantially to eliminate transfer to said input system of energy ofsaid other frequency. i

10. A superheterodyne receiving system comprising a first detector, aselector system in advance of said detector comprising a path includingan inductance and a variable condenser in series, and a loop tunable tothe desired signal comprising an inductance and a variable condenser,similar to and adjustable through substantially the same range ofcapacity as said first condenser, means mechanically coupling theadjustable elements of said condensers for movement in unison,

a local oscillator system, an intermediate frequency amplifier, and afixed condenser in said path in series with said inductance and variablecondenser thereof, and of such magni tude of capacity that said path,throughout the range of adjustment of said condensers in unison, iscontinually tuned to series resonance for a frequency higher than thefrequency of the desired signal by twice the in termediate frequency. I

11. A superheterodyne receiver comprising a first detector, aninputsystem therefor including a variable condenser, an open antennasystem coupled to said input system and including a path having aninductance and a variable condenser in series, a local oscillator systemhaving a variable condenser, an intermediate frequency amplifier, andmeans mechanically coupling'said condensers for movement in unison totune said input system to a desired signal frequency, to generatelocaloscillations whose frequency differs from the desired frequency bythe intermediate frequency, and simultaneously to effect" seriesresonance of said path for another frequency differing from the desiredsignal fre- V quency by twice the intermediate frequency substantiallyto eliminate transfer to said input system of energy of said otherfrequency.

12. A superheterodyne receiver comprising a, detector, an input systemtherefor including a variable condenser, an open antenna system coupledto said input system and" including a pathlhaving an inductance and'avvariable condenser in'series, a local oscillator, an intermediatefrequency amplifier, and means mechanically coupling said condensers formovement in unison to tune said'input system to a desired signalfrequency andsimultaneously to efiect series resonance of said path foranother frequency differing fromfthe desired signal frequency twice theintermediate frequency substantially'to eliminate transfer to saidinputsystem of energy of said other frequency.

, VERNON o. AGNABBQ

